In Bleach, why does Yhwach/Juhach call Ichigo his son?
Yhwach is Ichigo's father in spiritual meaning. He was the first Quincy and he has the ability to give a part of his power to others. By giving that power, the other person would turn into a Quincy as well. Long story short, those Quincies married, and had offsprings. Since Yhwach was the one who made them Quincy, it can be said that Yhwach is the one who created them.
It was said that the name Yhwach is taken from the name of Israelites God, Yahweh. In Abrahamic religions, God is said to have created man from dust and made them according to his image. In Christian and Catholic belief, man are said to be children of God.
The same thing is applied to Yhwach's case. Since he was the one who created the Quincy, all Quincies can be said to be his children, that is in spiritual sense. Since Ichigo's mother was a Quincy, Ichigo too is Yhwach's son.